loose tubes

As long as the pins are making good contact and the tubes do not "vibrate" out, it should be fine (SEE DISCLAIMER BELOW). I do the same thing to the tubes in my Audion Silver Night (both signal and output) and use the tip of a thin fingernail file (power is off) to gap them so that they do not ride too high. Since you mention it, I am just telling you that you are not alone in your observation, but I do not recommend this to people in the event that a tube will vibrate out and cause damage to their equipment (my tube sockets are in good condition and grip very tightly, so I don't worry about it). I had thought of using a thin Sorbothane pad (with holes for the pins) between the tubes and the sockets (thought that this might act as kind of a damper), but have not yet tried it out and am guessing that Sorbothane does not conduct current which would have to be verified first.

This is truly fascinating. Has anyone an explanation for this phaenomenon?

I recently had a loose connection in one of my output tubes in one of my monoblocks and it arced and shorted out some equipment upstream from it. The fellow who repaired my equipment thought it was the loose connection that caused it.(I recently moved the monoblock when I repositioned my speakers). As a long time tube person I know that it could of arced anyway but now I always make sure my output tubes are fully seated.

Detlof: I think the difference in sound just comes from lifting the glass tube globe (the bottom of it on my signal tubes) and the plastic tube base (on my output tubes) up off of the tube socket. My tube sockets have slightly raised centers. This way vibration is only transmitted by the pins (instead of both the pins and the bottom of the tube. Rec makes a very good point that by raising the tubes you are then exposing the pins themselves. Again, although I practice this myself, I do not recommend it because of the risks involved. My tubes are also not enclosed and are easy to check and monitor. I cannot hear a difference when I raise the 5687 driver tube in my amp (but I do it anyway). The 6922 types though do sound better to me (more detail though at the same time a little smoother on top - kind of hard to describe) and the 300B's seem to focus better and play a bit louder while still maintaining clarity. My tube amp is enclosed in a closet off the living room (where the speakers are located) and receives very little air born vibration and this may be why this additional tweak is even audible to me at all as it is not a major change in sound, but a change (that I like) nonetheless.